Viologen-based redox electrolytes are recognized for their two-electron redox process, and low self-discharge
characteristics without ion-exchange membrane. However, their electrochemical performance has not yet
been fully realized, as only the first redox couple, i.e., dicationic species (V2+)/monocation radical (V+˙), is
typically utilized in practical energy-storage devices. The second redox couple, corresponding to the V+˙
/neutral
dialkylbipyridinylidene(V◦) is generally not realized because of low electrochemical window of the aqueous
electrolytes, the poor solubility of V◦, and its tendency to undergo dimerization and aggregation in the non￾aqueous electrolytes. To address these limitations, molecularly engineered viologen derivatives containing
permanently charged substituents or extended π-conjugated moieties have been developed to increase the sol￾ubility of V0 and reduce its dimerization and aggregation . Nevertheless, several studies have reported stable
utilization of the second redox couple even for simple dialkyl viologen derivatives. These seemingly contra￾dictory observations necessitate a systematic re-examination of the accessibility, reversibility, and long-term
stability of the second redox couple in viologens. In the present work, we have systematically evaluate the
beaviour of diethyl and diheptyl viologen derivatives with bromide and hexafluorophosphate counnterions in
acetonitrile (ACN), ionic liquid (IL) and their mixtures. Electrochemical investigation reveal that the irreversible
redox failure of these redox additives in ACN and IL/ACN, highlights the limitations of simple viologen de￾rivatives. A possible mechanism for the observed failure is proposed based on the synergistic correlation between
zero-charge voltage, the potential attained by the negative electrode during electrochemical measurement in the
full cell, and the stability of the two redox couples of the bipyridinium ion, as determined from half-cell mea￾surement. Additionally, FTIR, UV, and XPS analyses were employed to provide evidence supporting the proposed
mechanism.